A leaking heater core would cause fogging. Is there a faint smell of
antifreeze inside the car? You could probably exaggerate the smell (if a leak
is the problem) by turning the heat on full blast. This happened in my old
'86 coupe and my dad's '85 VW golf. I think I remember that there was a
recall for the VW's to replace the heater core with a new improved version. I
don't know if Audi issued a similar recall.